The present invention relates to an optical head device of the type in which a laser light is emitted from a laser light source and reflected on an optical recording medium, and a resultant return light passes through or is reflected by a plurality of optical elements, and is led to a light receiving element.
CD, CD-R, DVD and the like which are different in substrate thickness and recording mode, e.g., recording density, are own for the optical recording medium. To reproduce information from a DVD containing information recorded thereon in high density, it is necessary to use a short-wave laser light having a wavelength of 650 nm or 635 nm. It is a common practice that a laser light having a long wavelength of 760 to 780 nm is used for the CD reproduction. However, the short-wave laser light for the DVD reproduction may be used for the CD reproduction, as a matter of course. The CD-R (recordable) or the CD-RW (rewritable), which is developed on the basis of the CD, is designed so as to produce the maximum performances by using the long wave laser light, which is generally used for CD reproduction. Therefore, to handle both the CD-R and DVD by a single optical head device, it is necessary to install two laser light sources to the device, a first laser light source for emitting a short wave laser light, and a second laser light source for emitting a long wave laser light.
If two separate optical systems are used for the optical head device, the number of optical elements is increased when comparing with the optical head device using a single optical system. Further, the device size is increased, and the device cost is also increased.
To cope with this, a related optical head device, as shown in FIG. 5, uses a first light source (first laser light source) 4 for emitting a first laser light (short wave laser light) L1 having a wavelength of 650 nm and a second light source (second laser light source) 5 for emitting a second laser light (long wave laser light) L2 having a wavelength of 785 nm. The first laser light L1 emitted from the first light source 4 and the second laser light L2 emitted from the second light source 5 are guided, by a prism 106 serving as an optical path composition element, to a common optical path 10 destined for the optical recording medium. In the optical system, a mirror 11, a collimate lens 12 and an objective lens 13 are disposed in this order on and along the common optical path 10.
In the optical head device 101, to guide the first laser light L1 emitted from the first light source 4 and the second laser light L2 emitted from the second light source 5 to the common optical path 10, a half mirror 107 serving as a return light splitter is disposed on the optical path ranging from the second light source 5 to the optical recording medium. The half mirror 107 partially reflects the second laser light L2 emitted from the second light source 5 toward the prism 106, and allows a return light from the optical recording medium to partially pass therethrough so that the return light is directed to the light receiving element 9. The first laser light L1 emitted from the first light source 4 is directly incident on the prism 106.
In the optical head device 101 thus constructed, for the DVD first laser light L1, of the first and second polarized light components of which polarization directions are perpendicular to each other, the first polarized light component is used. For the CD second laser light L2, the second polarized light component is used. Assuming that the first and second polarized light components are respectively the S- and P-polarized light components in the prism 106 and the half mirror 107, the partial reflection faces 160 and 170 of the prism 106 and the half mirror 107 exhibit optical transmittance indicated by solid lines P and dashed lines S in FIGS. 6A and 6B, for the S-polarized light component and the P-polarized light component.
Accordingly, the first laser light L1 emitted from the first light source 4 is first incident on the prism 106; a light component which is substantially the half of the laser light is reflected by the partial reflection face 160 of the prism 106; the optical axis of the laser light is curved by 90 degrees and the laser light is directed to the mirror 11; and the laser light is reflected upward and incident on the collimate lens 12. The first laser light L1 thus guided to the collimate lens 12 is converted into a collimated light beam, guided to the objective lens 13, and converged into a light spot on the recording face of the DVD as an optical recording medium, by the objective lens 13.
The first laser light L1 as reflected by the optical recording medium travels back through the objective lens 13, the collimate lens 12 and the mirror 11, and reaches the prism 106. A light component of approximately 50% passes through the partial reflection face 160 of the prism 106 and advances to the half mirror 107. Most of the return light of the first laser light L1 passes through the partial reflection face 170 of the half mirror 107; it is incident on the sensor lens 15; it passes through the sensor lens 15; and it reaches the light receiving element 9. Accordingly, a light component of approximately 50% of the first laser light L1 emitted from the first light source 4 is guided to the optical recording medium, and the light component of approximately 50% of the return light of the first laser light L1, which is returned by the optical recording medium, reaches the light receiving element 9.
The second laser light L2 as emitted from the second light source 5 is incident on the partial reflection face 170 of the half mirror 107, and a light component of approximately 50% of the incident laser light is reflected by the partial reflection face 170. The optical axis of the reflecting light is curved by approximately 90 degrees and the light is incident on the prism 106. Most of the second laser light L2 that is incident on the prism 106 passes through the partial reflection face 160 of the prism 106, and is directed to the mirror 11 on the common optical path 10. Then, the light is reflected upward by the mirror 11 and goes to the collimate lens 12. The second laser light L2 that is thus guided to the collimate lens 12 is converted into a collimated light beam, and then guided to the objective lens 13. The laser light is converged into a light spot on the recording face of the CD as the optical recording medium, by the objective lens 13.
The second laser light L2 that reflected by the optical recording medium travels back to the prism 106, through the objective lens 13, collimate lens 12 and mirror 11. Most of the second laser light passes through the partial reflection face 160 of the prism 106, and goes to the half mirror 107. AS light component of approximately 50% of the return light of the CD laser light L2 passes through the partial reflection face 170 of the half mirror 107; it is incident on the sensor lens 15; and it passes through the sensor lens 15 and reaches the light receiving element 9. Accordingly, a light component of approximately 50% of the second laser light L2 that is emitted from the second light source 5 is guided to the optical recording medium, and the light component of abut 50% of the second laser light L2 reflected by the optical recording medium reaches the light receiving element 9.
The construction of the optical head device described above is valid on the assumption that the optical recording medium does not have birefringence, and when the first laser light L1 and the second laser light L2 are reflected by the optical recording medium, the polarization planes of them are not varied. Many optical recording mediums, commercially available, have birefringence in the radial direction since in the process of resin molding the optical recording medium, the formation of the optical recording medium is affected by the resin flow direction. For this reason, when information is reproduced from the commercially available, optical recording medium by use of the related optical head device, satisfactory reproducing characteristics of the optical head device can not be always obtained.
In the light that is incident on and reflected from a disc having birefringence, a phase difference is caused between a polarized light having an ordinary ray direction by the birefringence and a polarized light having an extraordinary ray direction. When the polarization direction of the first polarized light and the direction of the second polarized light are not perpendicular to the ordinary ray direction and the extraordinary direction, a light-amount ratio of the first polarized light and the second polarized light before those are incident on the disc is different from that after those are incident on the disc. Accordingly, in the worst case, there is a case that when only the first polarized light is incident on the disc, the reflecting light is entirely changed into the second polarized light.
In the optical head device thus constructed, when the first laser light L1 is reflected by the optical recording medium D, and the S-polarized light component is changed into the P-polarized light component through the action of the birefringence of the disc, a transmittance of the laser light on the partial reflection face 170 of the half mirror 107 is 100% and remains little changed. However, the transmittance of the laser light on the partial reflection face 160 of the prism 106 is reduced to approximately 20% of the original one. Therefore, of the return light of the first laser light L1 that is reflected by the optical recording medium, only the light component of approximately 20% of the return light reaches the light receiving element 9.
When the second laser light L2 is reflected by the optical recording medium, and its P-polarized light component is changed to an S-polarized light component, the transmittance of the second laser light on the partial reflection face 160 of the prism 106 is reduced to approximately 80% of the original one, and the transmittance of the laser light on the partial reflection face 170 of the half mirror 107 is reduced to approximately 30%. As a result, the light component of only approximately 24% of the return light reflected by the optical recording medium reaches the light receiving element 9.
In order that when the first laser light L1 and the second laser light L2 are reflected on the optical recording medium, good reproduction characteristics are secured even if the polarization plane of each laser light is changed because of birefringence of the optical recording medium, a measure may be taken in which the partial reflection faces of the prism and the half mirror are of the non-polarization type. The transmittance exhibited by such partial reflection faces of the prism and the half mirror are shown in FIGS. 7A and 7B, respectively. The partial reflection faces of the prism and the half mirror are formed by layering a multiple of dielectric films. To form the partial reflection faces of the non-polarization type, at least 30 number of dielectric layers must be layered. The prism and the half mirror having the partial reflection faces so structured are expensive. Those layers have manufacturing variances, and hence the transmittance and reflectivity characteristic variances of those optical elements are large with increase of the number of layers.